Agonists and Antagonists

Examples from medicine labels

The table uses wording from US prescribing information (the "label"), which is the regulator-reviewed description of what each approved medicine does.

An agonist can end up turning a system down

The leuprolide row looks odd: an agonist that lowers hormones. The label explains it. After the first administration, leuprolide causes an initial rise in luteinizing hormone, follicle-stimulating hormone and then testosterone, followed by sustained suppression. The label reports testosterone falling to castrate levels about 21 days after starting. [4] Degarelix, an antagonist at the same receptor, blocks it from the start. [5] Both labels describe effects in prostate cancer patients, which is a specialist setting. This is one reason "agonist" and "antagonist" cannot be read as "good" and "bad", or "up" and "down". What happens depends on the molecule, the dosing pattern and the tissue.

More shades than on and off

• Partial agonists switch a receptor on only partly. Buprenorphine, a non-peptide pain medicine, is described as acting through partial and biased agonism at the mu-opioid receptor. [6]
• Biased agonists favor some signals from a receptor over others. A pharmacology primer notes that the heart medicine carvedilol is a biased agonist at beta-adrenergic receptors. [7]
• Dual agonists act on two receptors. Tirzepatide is the labeled example above. [2]

These examples are not peptides, but the ideas apply to peptide signals too. Each receptor target has its own detail, so the table is a guide to vocabulary, not a rule.

Why the words matter when you read about peptides

Peptide drug research is full of receptor language. A review of peptide medicines for metabolic disorders notes that peptide agonists acting on G protein-coupled receptors are used in therapy for diabetes and obesity. [8] Marketing for unapproved products borrows the same vocabulary.

• A receptor claim is a mechanism claim. "Activates a receptor" tells you what a molecule does in a lab setting. It does not tell you what happens to a person's health. See mechanisms vs demonstrated clinical benefits.
• Binding is specific to the molecule. A similar-sounding name may not bind the same way. See why similar names do not mean equivalent products.
• Switching a receptor on or off has consequences elsewhere. The same receptor can sit in several organs, so effects are often broader than the one people hope for. Safety is specific to each compound; see peptide safety and adverse events.